Non-ionizing radiation: RF & Microwaves. Mike Yost, MS PhD Radiation Safety: Principles, Practice, and Emerging Issues

Transcription

1 Non-ionizing radiation: RF & Microwaves Mike Yost, MS PhD Radiation Safety: Principles, Practice, and Emerging Issues

2 Section #1: Current Issues + Some hot topics RF, cell phones & cancer NTP animal study of RF and cancer Interphone Epi study of mobile phones & cancer RF dosimetry & Bioeffects Brain dose for different ages Endpoints other than cancer

3 Topic A: Mobile Phones & Cancer Video March 26-28, 2018: Peer Review of the Draft NTP Technical Reports on Cell Phone Radiofrequency Radiation

4 Mobile phone subscriptions per capita in the US, by year Little M P et al. BMJ 2012;344:bmj.e1147

5 RF and Cancer q Evidence on health effects and exposure to ELF & RF EMF: o The IARC classified RF (2011) as possibly carcinogenic to humans (Group 2B) based on epidemiologic studies of exposure to mobile phones and brain cancer o Evidence for brain cancer from occupational RF exposure was judged inadequate q Why inadequate for occupational? o Limited exposure assessment in previous studies based on surrogate measures, expert judgement, but: Ø No exposure measurements! o Previous studies based mainly on JEMs that apply a job s average exposure to all individuals with that occupation Ø Berkson error increases uncertainty

6 The Interphone Study: Interview with Dr. Elizabeth Cardis Living on Earth Radio Program, May 21, 2010 Play the interview INTERPHONE Study Group. Brain tumor risk in relation to mobile telephone use: results of the INTERPHONE international case-control study. Int J Epidemiol 2010; 39:

7 Table 3: ORs between mobile phone use and brain tumors by cumulative call time, stratified by recency of starting regular use - excludes use with hands-free devices Meningioma Glioma Cases Controls OR a (95% CI) Cases Controls OR a (95% CI) Cumulative Call time (h) Non-regular users Short-term users: start of phone use 1 4 years before reference date <5 h ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) Medium-term users: start of phone use 5 9 years before reference date <5 h ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) Long-term users: start of phone use 10 years before reference date <5 h ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )

8

9 What is already known on this topic q The IARC recently re-evaluated brain tumor risks associated with mobile phone exposure and classified microwave radiation produced by mobiles phones as a possible human carcinogen, largely based on relative risks reported by two epidemiological studies, the 2010 Interphone study and a 2011 Swedish study by Hardell and colleagues. q However, trends in USA brain cancer incidence have not mirrored the substantial increase in mobile phone use since the mid-1990s, and have generally remained constant. q Based on relative risks from the (Hardell) Swedish study, predicted rates of glioma in the USA were much higher than observed rates. However, based on relative risks from the Interphone study, projected rates could be consistent with the observed data

10 Estimates of RF absorption into the brain based on age (courtesy from ). *Risk of Brain Tumors from Wireless Phone Use Dubey, RB et.al, Journal of Computer Assisted Tomography. 34(6): , Copyright 2012 Journal of Computer Assisted Tomography. Published by Lippincott Williams & Wilkins. 10

11 Example of bio-effects study Effects of Cell Phone Radiofrequency Signal Exposure on Brain Glucose Metabolism Nora D. Volkow, MD; Dardo Tomasi, PhD; Gene-Jack Wang, MD; et.al. JAMA. 2011; 305(8):

12 Current assumptions that may be important. Increasing phone usage or prevalence = increases in exposure Usage of phones is a good proxy for exposure to RF Dose response is monotonic Health effects are same regardless of age or history of prior usage Nature of RF Technology change does not change exposure over time (Possible role of bias in observations?)

13 Lasers in the News Aircraft Lasers Strikes October 22, 2015: ferry master and chief mate of the Tokitae injured with a blue laser Still no charges for man accused of injuring ferry employees with high-powered laser ( 6 mos ) Whidbey Island man charged for injuring ferry captains with laser Washington man fined $100K for pointing laser at ferry, injuring 2 crewmembers Laser lights still a problem for Oregon pilots (Seattle times: 11/17/17)

14 Section #2: RF - Introduction & Concepts

15 Radiant (Electro-magnetic) Energy Non-ionizing Radiation Not capable of ionizing tissue (~water or DNA) Ionizing Radiation IS capable of ionizing tissue (DNA)

16 Electric Charges create Electric Fields

17 Electric Fields represent the force on a charged particle or objects nearby The amount of charge is proportional to the voltage Polarity (+/-) determines the direction of force: opposites attract! Force depends on the distance between the charges and the amount of charge on the objects: Inverse Square Law (Coulomb s Law) é Click 4 Video

18 Magnetic Fields Invisible lines of force that start at the north pole of a magnet and come back around, land on the south pole of the magnet, and complete a circuit by going through he body of the magnet to reemerge at the north pole. ç Click 4 Video

19 Magnetic fields are created by moving charges (current flows: Ampère's law) + F=qUB sin θ Magnetic fields exert a force on other moving charges nearby

20 Magnetic Fields are proportional to currents Moving electric charges create magnetic fields Field lines form closed loops around a wire Direction of field lines depends on current flow Magnetic field lines Conventional current flow Right hand rule

21 Connections of electricity and magnetism Click 4 Video Demo Induced Electric potential Current flow Induced Current flow Induced Magnetic field Changing Magnetic field

22 Magnetic Fields from Alternating currents When the polarity and strength of the current and voltage keeps changing between positive and negative, we call this an alternating current AC The frequency of the AC describes the number of times per second that the current undergoes a complete (+/-) cycle. Frequency = Hz = cycles/sec AC creates alternating electric and magnetic fields Example: in USA, polarity of voltage and current flowing in power lines changes 60 times per second.

23 AC Current and Voltage relationships Ohm s Law V = IR Joule s law P = IV V=Voltage; I = Current; R=Resistance P=Power

24 Time varying fields can have special properties: Electromagnetic Radiation Maxwell s equations: electric and magnetic fields can act together as to create a self propagating wave Electromagnetic radiation Changing Electric Field Induced Electric Field Induced Current Flow Changing Magnetic Field

25 Electromagnetic Radiation Magnetic and Electric Fields change polarity and strength rigidly bound together Field is pointing in some direction, the magnetic field must move in a direction perpendicular to the electric field the wave must travel in yet another direction perpendicular to the other two

26 Two field components in a free propagating electromagnetic wave Electric Field (E) Perpendicular to direction of travel Units of Volts/meter Magnetic field (H) Perpendicular to E and direction of travel Units of Amps/meter Combined fields carry power S over space (Amps/m) x (Volts/m) = Watts/meter 2 E x H = S

27 Recap: Electromagnetic Radiation C = speed of light ~ 300 x 10 6 m/s (in vacuum) Period = p = the amount of time elapsed during one cycle Frequency = f = the inverse of the period = # cycles/sec Wavelength (λ) the distance the wave travels in one cycle. Frequency *wavelength must equal C = speed of light as frequency increases, wavelength decreases

28 Dual nature of electromagnetic energy: waves and photons Photons are packets of electromagnetic energy Photon energy is directly proportional to frequency Photon energy = J = h f J has units of Joules or electron-volts (ev) f = frequency in cycles/second (Hz) h = planck s constant (4.144 x ev/hz) Recall C = f λ= 3 x 10 8 m/s (speed of light) Ionizing radiation has J > 10 ev 10 ev approx. threshold for ionizing tissue/water

29 Electromagnetic spectrum Arrange EM radiation in order of energy This defines regions of related EM waves Increasing frequency related to higher energy Ionizing vs Non-ionizing boundary in the UV

30 The Electromagnetic Spectrum Frequency Band Wavelength EV Click 4 NASA Video

31 Radiofrequency Region Frequency Band Wavelength EV

32

33 Absorption causes heating due to energy deposition Energy transfer depends on wavelength, polarization and geometry

34

35

36 Analogy to a light source Color frequency of RF emission Bulb wattage power of the RF source Light intensity at a distance power density Warming SAR

37 RF Terminology Frequency = Cycles per second (Hertz) Power = Rate of energy transfer (watts) Power density = Exposure at body surface = Power/surface area (Watts/meter 2 ) SAR = Absorbed dose in tissue = Absorbed power/body mass (Watts/kg)

38 Free Field EM Plane wave propagation (1) In the far/free field: E H Propagation to both E and H; (No field in direction of propagation) Speed = speed of light in medium 1/ (εµ) i.e. depends on permittivity (ε) and permeability (µ) of the medium For biological materials, permittivity (ε) dominates In a vacuum: ε 0 = x F/m (Farads per meter) µ 0 = 4π 10 7 V s/(a m) (Newtons / A 2 )

39 Free Field EM wave propagation (2) v Wavelength (λ) is speed / frequency v Ratio of E/H is characteristic impedance Z v Z approximately constant for air v Power in free space (Poynting s Vector) v Power = S = E H = E H sin θ v Z = E/H = 377 ohms in free field v or S = E 2 /377 = 377 H 2 This is in SI units!

40 How does RF transmission work?

41 Current in a Radio Antenna Current distribution in 2 nd half cycle

42 Radio wave propagation Click Here for a Video Summary

43 Far field power density approximation Power density = S = G P o /4π r 2 G is antenna gain, P is transmitter power Note G is a power ratio: the power in given direction compared to isotropic radiation (P o ) (often expressed in db = 10 log P/P o )

44 Antenna Power Gain Isotropic source Power density = S = P/4π r 2 Directional source Power density = S = G*P/4π r 2 Receptor located at a fixed distance

45 The NEAR field is Different! Near a source (antenna) E and H are not perpendicular Also, there are reflection and fields due to currents flowing in nearby objects Define far field as distance ~ r > 2L 2 /λ Where L is largest dimension of antenna

46 Near / Far Field regions S S Far field E/H = 377 ohms Approx. Near/Far boundary distance

47

48

49 Calculation Example: Horn antenna f= 300 MHz, P= 10 kw, λ = 1.0 meters, aperture L = 2 meters; G= 50 (17 db) Far field is distance > 2L 2 / λ (or 8 m) At distance of 30 meters from source S = 44.2 w/m 2 = 4423 µw/cm 2 Note: db Gain = 10 Log 10 (50) = 10 (1.699) 17 db

50 Short Break! Topics to review: Calculation and estimation of EM field power density from antenna power and distance Properties of electric, magnetic and EM fields

51 Quick Questions What is the power density for a 1kW source with an antenna gain (G) of 126 at a distance of 100m from the source (assume far field)? What is this antenna gain expressed in db? What would the power density be at 200m?

52 Quick Questions What is the power density for a 1kW source with an antenna gain (G) of 126 at a distance of 100m from the source (assume far field)? ANS: ~1 W/m 2 What is this antenna gain expressed in db? ANS: 21 db What would the power density be at 200m? ANS: 0.25 W/m 2

53 RF Exposure Bio-effects What s behind the standards

54 Factors in RF Bioeffects Absorption, Reflection, Transmission Absorption causes heating due to energy deposition Energy transfer depends on wavelength, polarization and geometry

55 Depth of penetration depends on wavelength: Wavelength (cm) skin depth 3 (10 GHz) <1 mm cm cm cm Polarization of EM wave also a factor

56 How RF Transfers Energy

57 Tissue electrical properties vary with frequency Dielectric Constant 10 5 ε σ Conductivity Frequency (Hz)

58 Polarization refers to the orientation of E or H relative to vertical /horizontal axis; usually E vertical is "worst case (circular polarization also common)

59 Body Resonance: when wavelength ~ body dimensions People act as good antennas when the wavelength is similar in size to the body Whole body coupling is to the vertical (long) axis or the body, vertical E field Higher frequencies can couple to body parts; head arms legs, etc. Similar frequency effects for children.

60 Specific Absorption Rate Absorbed power in Watts/kg SAR vs. Frequency

61 Body Resonance and SAR Sub-resonant: (<3 MHz) Body and parts don t act as antennas. Current flows dominant, SAR proportional to f 2 Transition: (<300 MHz 6 GHZ) absorption drops 1/f Avg. SAR W/Kg Resonant: (3 MHz 6 GHz) Body and parts act as good antennas, both current flows (, 100 MHz) and local absorption important. Absorption increases in proportion to f 2 from 3 to 30 MHz. Max MHz. Bounding Envelope Super-Resonant: (6 GHz 300GHz) Body does not act as an antenna; absorption constant with f, approx 8% of incident power. Quasi optical focusing 6-15 GHz; skin absorption dominant > 15 GHZ

62 SAR = absorbed dose SAR measures the energy transfer per unit body mass, Depends on many things (shape polarization, frequency, etc.) SAR = E 2 σ/ρ σ = tissue conductivity (siemens/m) ρ = tissue density (kg/m 3 )

63 Summary of biological effects: SAR and power density Tissue energy absorption proportional to incident EM power/unit area Power density = Exposure in Watts/m 2 or mw/cm 2 S = (Poynting vector) is a measure of this incident energy Exposure used for setting external exposure limits SAR measures a bio-effect(s) of dose (e.g. heating)

64 Energy transfer or dosimetry is related to SAR absorbed dose; SAR = power absorbed/tissue mass or Watts/kg where SAR is the "specific absorption rate" SAR used to set internal dose limits (basic restrictions on exposure) SAR = E 2 σ/ρ σ = tissue conductivity (siemens/m) ρ = tissue density (kg/m 3 )

65 Protective clothing

66 Personal Alarm

67 Cooperative RF Program for Shared Tower

68 Cooperative RF Program for Multiple Broadcasters

69 Fence to Limit Access

70 RF Protective Suits

71 PPE Must Be Tested for Application

72 PPE Must Be Inspected & Maintained From Ric Tell

73 END Bonus slides & extra material follow

74 Note some in CGS units!!

75 Example: ICNERP MPE

76 Induced current limits (whole body) Induced Current limits for each foot or grasping contact: I 1000/f where 0.03<f<0.1 MHz {30 khz to 100 khz} I 100mA for 0.1<f<100 MHz {100 khz to 100 MHz} Note: 1 s averaging time Pulsed fields <100mS 0.1kHz to 15 GHz Peak TLV=TLV*360/ 5*pulse-width Pulse width in seconds

77 Summary of Contact Current

78 Antenna Equations